1. Field of the Invention
The present invention relates to a method for blasting in bar-like charge, more particularly, relates to a method for blasting employing bar-like charge being capable of ensuring a blasting under safety condition without causing danger of flying rock.
2. Description of the Related Art
A number of accidents by blasts for construction works in Japan from 1979 to 1989 are counted 261, in which accidents by flying rock resulting from blasts are counted 160 cases, which are 61.3%.
In the prior art, in a single point concentrated charge system for blasting as shown in FIG. 1, a charge amount (L) of an explosive is expressed by Hauser's equation: EQU L=cW.sup.3 ( 1)
wherein
L: charge amount (kg) PA1 c: blasting coefficient PA1 W: line of least resistance (m) PA1 D1: distance between the blast holes E and A; PA1 D2: distance between the blast holes E and B; PA1 L: charge amount of explosive; and PA1 v: fracture rock volume "H.times.D1.times.D2" corresponding to the charge amount L.
From the foregoing Hauser's equation of blasting, the blasting coefficient c can be expressed by: EQU c=L/W.sup.3 ( 2)
The foregoing Hauser's equation of blasting is established under the following conditions:
1. Full charge amount L is charged in the single point concentrated charge system;
2. The blasting is a single freedom surface blasting. and
3. The proper charge amount, namely the charge amount for obtaining maximum fracture effect within a safety range, in which flying rock or scattering stone will not be caused, is determined with respect to funnel shape blasting configuration of W=r, in which the fracture radius r on a free surface G is equal to the line of least resistance W.
Accordingly, the volume V of the funnel hole is expressed by: EQU V=1/3.times..pi.r.sup.2 .times.W
Here, from the condition of W=r as set forth above, and since .pi..apprxeq.3, the foregoing equation (3) can be modified as: EQU V=W.sup.3 ( 3)
By replacing W.sup.3 in the foregoing equation (2) with V in the equation (3), the equation (2) can be expressed by: EQU c=L/V (4)
As can be appreciated herefrom, the blasting coefficient c is a ratio of the single point concentrated charge amount L versus the fracture volume of the rock with the charged explosive. The blasting coefficient c as set forth above can be established when the three dimensions Wr.sup.2 forming the fracture volume V are equal to each other. (see Japan Industrial Explosive Association, "NEW INDUSTRIAL EXPLOSIVE", Oct. 1, 1985, pages 198 to 200)
On the other hand, in simultaneous blasting with bar-like charging system as shown in FIG. 2, the charge amount L is expressed by: EQU L=c.times.H.times.D1.times.D2 (5)
By modifying the foregoing equation (5), the blasting coefficient c is expressed by: EQU c=L/(H.times.D1.times.D2)=L/V (6)
Here, the relationship between the distances D1 and D2 between blast holes and the blast hole length H has to be: EQU (D1=D2)&lt;H
wherein
(see, "Explosive Safety Text Series 17, Application of Explosive in Occasions" edited by Ministry of International Trade and Industry, Ground Emission Division, published by Shadan Hojin Zenkoku Kayakurui Hoan Kyokai, January, 1991, pages 45 to 46)
This inventor's viewpoint is as follows; namely, the blasting coefficient c represents the fracture force acting on free surface G. In other words, the blasting coefficient c represents the degree of upward force along the least resistance line W toward the free surface from the upper end of the charge length N.
Accordingly, when the charge amount L is to be determined, in excessive consideration is given for safety so as not to cause flying rock or scattering stone, the fracture force becomes excessively small to degrade efficiency of the blasting operation. Conversely, when excessively high efficiency is attempted by increasing the charge amount, it may cause flying rock to cause danger. Therefore, in order to optimize blasting operation, it is essential to properly determine the blasting efficient c in view of the balance of the safety and efficiency of the blasting operation, so that the maximum rock fracture can be obtained within a safety range, in which the flying rock may not be caused.
In reviewing of the blasting coefficient c derived through the conventional method in viewpoint set forth above, it should be true that, in the single point concentrated charge system, for which the Hauser's equation of blasting is applicable, since all of the fracture force necessary for fracturing the rock volume V=r.sup.2 W is a fracture force acting on the free surface, the volume V per se is the pure value forming the denominator of the value c of the blasting coefficient. (see foregoing equation (4))
However, in case of the blasting with bar-like charge system, it is not true that the fracture force necessary for fracturing total rock volume V=H.times.D1.times.D2 is the fracture force acting on the free surface. (see the foregoing equation (6))
Namely, the total fracture rock volume V=H.times.D1 and D2 is a sum of the rock volume fractured by the upward force toward the free surface G and the rock volume fractured by the force which contributes only force fracturing lower rock without contributing upward fracturing toward the free surface G. Therefore, the pure blasting coefficient c has to be determined with the denominator corresponding to the rock volume fractured only by the upward fracturing force toward the free surface. In this regard, the rock volume to be fractured by the downward fracturing force which does not contribute for upward fracturing, has to neglected.
Therefore, in the foregoing equation (5), the value c called as the blasting coefficient in the blasting operation with bar-line charge system, cannot be a pure value, but, in practice, a fracturing rock unit indicative of the ratio of the total fracturing rock volume V. Assuming this value as k for the illustration, the foregoing equation (5) can be expressed by: EQU L=k.times.H.times.D1.times.D2 (5a)
and similarly, the foregoing equation (6) can be expressed by: EQU k=L/(H.times.D1.times.D2) (6a)
As set forth above, the blasting coefficient c derived through the conventional method, contains an error in determination of the volume forming the denominator value. Namely, since the calculation is performed with including the element which should not be associated with derivation of the blasting coefficient c, reference values are set at 0.10.about.0.30 (see page 46 of foregoing "Explosive Safety Text Series 17, Application of Explosive in Occasions") which are much smaller than typical proper blasting coefficients 0.25.about.0.45.
However, if those skilled in the art who is not knowledgeable about the uncertain element in derivation of the blasting coefficient c in the bar-like charge system in the conventional manner, applies the typical proper value of the blasting coefficients 0.25.about.to 0.45 as element for deriving the charge amount of the explosive, it can be feared on causing flying rock to make blasting operation dangerous.